1. Field of the Invention
This invention relates to fuel elements for nuclear reactors and, more particularly to fuel element end fittings that engage the ends of the fuel rods, and the like.
2. Description of the Prior Art
To produce useful power from a nuclear reactor, it is customary to load a suitable concentration of fissionable uranium in a group of hollow rods. This group of loaded rods, frequently referred to as the reactor core, transfers the heat that is generated through the fission processes in the uranium to water that is flowing through the core.
The group of rods that form the reactor core ordinarily are arranged in units that are called fuel elements. This sub-grouping of the fuel rods into fuel elements tends to enhance the structural integrity of the reactor core and simplify the tasks of initially loading the reactor core with fissionable fuel, rearranging partially used fuel within the core to obtain maximum fuel utilization and removal of the fuel for inspection, repair and replacement, as necessary.
In the reactor core, the fuel rods are exposed to a hostile environment that not only includes intense radiation, high temperature and fission product generation, but also includes the mechanical stresses and potential vibrations that are imposed by the water flowing through the core. To stabilize the individual fuel rods within the respective fuel elements, expensive and elaborate end grid and end fitting structures have been devised to enable the fuel rods to be essentially clamped between the end fittings. The end grids, moreover, are not entirely satisfactory because the contact between the fuel rod and the grid tends to produce a type of erosion in the fuel rod surface that is known as "fretting". Further in this respect, the end grid and end fitting structures that have characterized the prior art also tend to decrease the water pressure as it flows through the reactor core.
To control the power generated within the reactor core, control rods are inserted into the core to regulate the intensity of the fission processes. Frequently, the control rods are received in tubes that guide the rods as they are moved into or out of the reactor core. In addition to guiding the motion of the control rods in the reactor core, these guide tubes often serve the additional function of joining the opposite end fittings together in order to clamp the fuel rods between the end fittings. To enable the guide tubes to carry out this function of clamping the fuel rods in their proper spatial relation, the ends of the guide tubes are fitted with special sleeves and nuts. This arrangement not only is expensive, requiring a large number of specially produced and carefully machined parts, but also increases the time required for fabrication and assembly.
Clearly, there is a need for a fuel element end fitting that overcomes the inadequacies that have characterized the prior art.